Caption: Omega Centauri (NGC 5139), the largest of the globular star clusters in our galaxy, optical image. Found in the constellation of Centaurus, its name of Omega Centauri designates it as one of the 'stars' of the constellation. It is 16,000 light years from Earth, and contains millions of stars in a volume about 150 light years across. Globular star clusters formed billions of years ago in the early history of the galaxy.